A wind turbine is an energy conversion system which converts kinetic wind energy into electrical energy for power grids. Specifically, wind incident on blades of the wind turbine generator (WTG) causes a rotor of the WTG to rotate. The mechanical energy of the rotating rotor in turn is converted into electrical energy by an electrical generator. As wind speed fluctuates, the frequency of the AC (alternating current) voltage/current generated by the generator varies. Therefore, it is common for a wind turbine to have a power converter to convert the variable frequency AC voltage/current from the generator to a substantially fixed frequency AC voltage/current suitable to be connected/transmitted to AC transmission/distribution networks or power grids via a transformer.
A full scale power converter based wind turbine usually includes a power converter having a generator side converter coupled to a grid side converter via a direct current (DC) link. The generator side converter regulates the power of the generator. This power passes through the DC-link, and is eventually fed to the grid through the grid side converter. The grid side converter regulates the grid side power. The DC-link is an intermediary stage in the power converter, and is maintained at a constant DC-link reference voltage during the operation of the wind turbine. This reference voltage may be 1100 volts, for example. Normally the reference DC-link voltage is allowed to vary in a given voltage range e.g. 1050 to 1150V. The same is true for the Doubly Fed Induction Generator (DFIG) systems where only a portion of the power from the generator passes through the power converter.
A wind turbine may be requested to shutdown, for example due to a fault in the grid, component failure in the turbine/wind farm or requested by the grid operator. During a turbine shutdown, the controls of the generator side converter and grid side converter are disabled, and the respective circuit breakers between the generator and the power converter, and between the power converter and the power grid are opened. At this time, the voltage at DC-link is left to decay naturally. The time of the DC-link voltage decay is dependent on the capacitance and resistance of the DC-link. It is only safe for a technician to work (e.g. repair the faulty converter or maintenance) on the power converter/turbine when the DC-link voltage has decayed to 50 volts or below. Typically, the default resistance provided across the DC-link is significantly high e.g. in the range of 10 k ohms, and hence the time needed for the DC-link voltage to decay to 50 volts may take a few minutes. With the trend of increasing turbine size, the time taken for the DC-link voltage to decay to 50 volts also increases accordingly.
Even if the DC-link voltage is allowed to decay naturally (for example after 5 mins), there is no guarantee that the voltage has reached a level which is safe for the technician to work on.
Hence it is desirable to provide a fast ramp down of the DC-link voltage of a wind turbine during shutdown.